feat: a sprint calendar

A Calendario tab lays the project's sprints out month by month as bands,
with the releases and the tickets due on each day. Owners and admins plan
a sprint by clicking the day it starts, and the whole plan downloads as an
iCalendar file. Tickets can be searched by due-date range.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/schema.gql

@@ -462,6 +462,12 @@ input IssueFilterInput {
   areas: [String!]
   assigneeId: ID
   categories: [StatusCategory!]
+
+  """Due on or after this day (YYYY-MM-DD)"""
+  dueFrom: String
+
+  """Due on or before this day (YYYY-MM-DD)"""
+  dueTo: String
   labels: [String!]
 
   """Only tickets in no sprint"""